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An American Lung Association study of air quality across the United States says San Diego’s air is getting steadily cleaner. But the nation’s eigth largest city got a grade of F because the public is still exposed to significant amounts of pollution, especially during the summer.

Greater San Diego ranks seventh nationally among metro areas in ozone pollution and 15th in short-term particulates, says the report, released today. The report compares data collected from 1996-98 and 2007-2009.

Most of the pollution comes from local roads and freeways, which are heavily congested with cars and trucks. The ships using the Port of San Diego also contribute to the creation of smog, as do the vehicles that serve the county’s large agricultural industry.

San Diego also gets some smog and particulate that drifts here from greater Los Angeles and Mexico.

"The grade of F is a wake-up call to remind people that while that air has been getting better in San Diego, there are still a lot of unhealthful air days that affect people who suffer from asthma and other lung problems," said Bonnie Holmes-Gen, senior policy director for the American Lung Association in California.

The ALA gives cities and counties a grade of F if the average amount of ozone and particulate exceed federal standards three days a year. In the mid-1990s, greater San Diego’s ozone levels were in the unhealthful range an average of 60 days a year, a figure that has since fallen to 29.5. The average number of days with unhealthful short-term particulates has dropped from 13 to 9. The particulate figure flucuates, rising during years like 2007, which San Diego County suffered devastating wildfires that sprewed particulate into the atmosphere.

Ozone pollution is typically worst in the summer, when smog is intensified by sunlight and heat. And onshore winds regularly cause some of the worst pockets of pollution to form at or near the base of local mountains.

Holmes-Gen said that greater San Diego, like many other areas, has been able to reduce the number of unhealthful days because, "Federal Clean Air Act and state regulations are making a difference. But we still have a long way to go."